1. Field of the Invention
The present invention relates to a method of measuring pressures using a tuning fork crystal oscillator.
2. Prior Art
There is a conventional method which allows for the use of a tuning fork crystal oscillator (which will be referred to simply as a "fork oscillator") to measure pressures (primarily pressures in vacuum), such as disclosed in Japanese utility model application, publication No. 64(1989)-38547. It is known that the fork oscillator has a resonance resistance which increases in proportion to pressures when a gaseous substance is in a molecule flow region, and increases in proportion to the square of half the pressures in a viscous flow region. The conventional method makes use of this property of the fork oscillator. It is also known that the fork oscillator provides a useful pressure measuring means since it is capable of measuring a wide range of pressures, i.e. from atmospheric pressures to pressures of 10.sup.-2 to 10.sup.-3 Torr.
According to the conventional method which uses the fork oscillator to measure such pressures, the fork oscillator is placed in the space of a particular gasesous substance whose pressure is to be measured thereby, and an oscillator circuit is used to cause the fork oscillator to produce oscillations. Then, the pressures in the gaseous substance may be determined from the difference .DELTA.Z (=Z-Z.sub.0) between the resonance resistance Z at that time and the natural resonance resistance Z.sub.0 of the fork oscillator (the value in high vacuum).
In the conventional method that uses the fork oscillator to measure pressures, as described above, the fork oscillator usually has a temperature that is indefinitely varying during the measuring process, which may cause large errors when measuring the pressures in the lower pressure range. This would inadvantageously make accurate pressure measurement impossible.
The natural resonance resistance Z.sub.0 as described above remains stable in a wide temperature range (-20.degree. C. to +60.degree. C.), such that it only changes by a factor of several K ohms (.OMEGA.) in that temperature range, while the value of the resistance difference .DELTA.Z is decreasing by the order of several K ohms in the pressure range between 10.sup.-1 and 10.sup.-2 Torr, and is decreasing by the order of several tens of ohms in the pressure range between 10.sup.-2 and 10.sup.-3 Torr, which means that the value of .DELTA.Z is decreasing as the pressures are reduced. The change in the natural resonance resistance Z.sub.0 that is caused by any changes in the temperature of the fork oscillator cannot be neglected. This therefore imposes limitations on the ability of the fork oscillator to measure the pressures.
In order to prevent the natural resonance resistance Z.sub.0 of the fork oscillator from being affected by any changes in its temperature, a pressure measuring probe, as shown in FIG. 6, is used which includes a fork oscillator 41 buried in an aluminum block 42, and a heater 43 and a temperature sensor 44 also buried in the same aluminum block 42 to maintain the fork oscillator 41 at a constant temperature.
For such a pressure measuring probe, however, the aluminum block 42 has an inherent thermal capacity that causes a time lag or delay to occur before the fork oscillator 41 is controlled to reach the specific temperature at which it should be maintained. Specifically, one problem of the device is the slow response time when the probe is measuring pressures, and another problem is that the measuring circuit must include an additional temperature control circuit for the heater 43 and temperature sensor 44, which makes the measuring circuit more complicated and expensive.